An implantable medical device such as a cardiac pacemaker supplants some or all of the natural pacing function of a heart by delivering appropriately timed electrical stimulation signals designed to cause the myocardium of the heart to contract. An implantable pulse generator (IPG) in the device generates the electrical stimuli. To be effective, the stimuli should be of a sufficient strength (or amplitude) and duration (or pulse width) to cause the heart to beat, i.e., to “capture” the heart. A “capture threshold” or “stimulation threshold” defined by a strength-duration curve separates stimuli that capture the heart from stimuli that fail to capture the heart.
Because a failure to capture the heart may result in serious complications or death, pacing stimuli are generally delivered with a strength and duration above the capture threshold by a safety margin. It is generally desirable, however, that the safety margin be reasonably large enough to ensure capture but small enough that power not be wasted. Implantable medical devices that draw power from a battery have a limited power supply, and the strength and duration of the stimuli should be regulated to prolong battery life. The strength and duration of stimuli may be set or adjusted with programming equipment that communicates with the implantable medical device.
Over time, the capture threshold can change. The change in capture threshold may be due to physiological factors, such as the physical condition of a patient or drug concentrations in the body of the patient. The threshold may also change due to non-physiological factors. For example, a lead that delivers stimuli to the myocardium may lose insulation or a filament in a lead having multifilar construction may break.
Because the capture threshold may change over time, a stimulation that may capture the heart at one point in time may fail to capture the heart at a later point in time. Accordingly, many implantable medical devices include the capability of automatically testing and measuring the capture threshold, in a process called “capture testing.” The devices may automatically adjust the strength or duration of the pacing stimuli as a result of the outcome of the capture testing. Conventional implantable medical devices measure the capture threshold on a substantially periodic basis, such as one time per day while the patient is at rest. The frequency of testing the threshold to assure capture is called the “capture test frequency.”